'Leap Second' Tonight Will Cause 61-Second Minute

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July will arrive a little late this year – one second late, to be
exact.

Time will stand still for one second this evening (June 30) as a
"leap second" is added to Coordinated Universal Time (UTC), the
time standard by which most clocks are regulated. The
International Earth Rotation and Reference Systems Service
(IERS), which keeps track of time for
the world, has decided that the extra second is needed to deal
with Earth's irregular but gradually slowing rotation.

The extra second will be inserted just before midnight UTC — just
before midnight GMT, and just before 8 p.m. EDT. Instead of
rolling straight through from 23:59:59 to 00:00:00, UTC will tick
over to 23:59:60 for a second. [ June
2015 Gets An Extra Second (Video) ]

The need for a leap second arises because of differences between
the time as recorded on our
atomic clocks and the time as recorded by the rotation of the
Earth in its revolution around the sun. But what is the reason
for this slowdown?

Recently, I had a chance to sit down with astrophysicist Neil
deGrasse Tyson, director of New York City's Hayden Planetarium.
We chatted about a number of different astronomical subjects, and
one of them was the impending leap second.

" The
moon is slowing us down," Tyson noted. "It's tugging on us.
If it ultimately succeeds at this, Earth's rotation will be as
slow as the lunar month, and we will always show the same face to
one another in what is called a 'double tidal lock.'"

"But," Tyson added, "if you do the math, it will take longer than
the lifespan of the sun for the moon to succeed at this. So it's
not something you should worry about at this point."

Along with the moon, other factors contributing to the slowing of
Earth's rotation include the sloshing of the planet's molten
core, the rolling of the oceans, the melting of polar ice and the
effects of solar gravity.

Since January 1972, timekeeping has, by international agreement,
been maintained in accordance with the atomic time scale. The
Earth is currently losing about three-thousandths of a second per
day, and, atomic clocks are just over six-tenths of a second fast
on UTC right now. The addition of the leap second will keep the
difference from exceeding nine-tenths of a second.

Excluding this evening's insertion, leap seconds have been added
25 times since 1972, most recently in June 2012.

Leap seconds are inserted, when needed, either on June 30 or at
the very end of the year, on Dec. 31. In 1972, there were two
leap seconds (in addition to that year also being a leap year).
From 1973 to 1979, adding a leap second on New Year's Eve was an
annual occurrence. But from 1999 to 2011 it was necessary to add
an extra second only twice (in December of 2005 and December
2008).